1. Field of the Invention
The present invention relates to a method of making high temperature, erosion resistant materials that are used in high temperature environments.
2. Background Information
Most components of combustion turbines are operated at very high temperatures and often require the use of specialized coatings/inserts to protect underlying supporting materials. These specialized coating/inserts include thermal barrier coatings (TBCs), turbine blade tip inserts, and abradable seals disposed opposite the turbine blade tips.
Conventional TBCs typically comprise a thin layer of a ceramic material covering an alloy. In many applications, the coatings must be resistant to extremely high temperature, erosion resistant. These materials may also be used as an abradable material. An example of an abradable material is turbine ring seal segments, which fit with tight tolerances against the tips of turbine blades, must withstand erosion and must preferentially wear or abrade in order to reduce damage to the turbine blades, and form a tight seal with the turbine blade. Protective coating system can include several layers including a metallic bond or barrier coating of MCrAlY having an alumina scale and, for example, a columnar yttria stabilized zirconia thermal barrier, which can be further coated by an erosion resistant layer of alumina or silicon carbide, applied by physical vapor deposition techniques.
Fillers have also been used. There, abradable stationary seal walls, for jet turbine housings which seal opposing, rotating rotor blade tips, have a ceramic core containing from 30 vol. % to 98 vol. % solid ceramic filler, where the ceramic fills a honeycomb wall structure. This is then covered with erosion and corrosion resistant outer layer, which is made porous by uniformly dispersed, finely divided filler. This is apparently applied by a spraying technique. The pores can be filled with ceramic, metal oxide or carbide materials. Fillers mentioned include hollow ZrO2.8YO3 ceramic spheres and solid Al2O3, SiC, TiC and BN spheres.
Other abradable honeycomb structures have been developed for use in turbines. Low melting fluorides, such as BaF2, are incorporated into a stabilized zirconia or alumina matrix which, in turn, is used to fill a honeycomb shroud lining made of, for example, a metal alloy. The filling becomes molten when the rotating blade tips rub the shroud, and upon resolidification, improve the smoothness of the abraded surface. Another variation of reinforced ceramic layers, including a honeycomb matrix for use in a turbine as abradable seals.
Also, U.S. Pat. No. 6,013,592 which is referenced here and is incorporated herein in its entirety, teaches a material containing hollow ceramic particles embedded in an interconnected ceramic matrix, to provide a composite thermal barrier coating system having superior erosion resistance and abrasion properties for use on combustion turbine components. The hollow particles were preferably spherical and made of zirconia, alumina, mullite, ceria, YAG or the like, having an average particle size of about 200 micrometers (0.2 mm) to 1500 micrometers (1.5 mm). The steady state erosion rate, (grams lost/kg of erosive impacting media or g/kg), of this filler was 3.2 g/kg vs. 4.6 to 8.6 g/kg for conventional TBCs.
The present invention is a method of forming an insulating material. And more specifically for forming the insulating material into larger three dimensional geometric shapes, such as cylinders. Here, the insulating material is formed by providing a permeable structure that has a contacting surface. A fibrous material is placed against the contacting surface of the permeable structure and then the hollow ceramic spheres are placed against the fibrous material. A slurry mixture containing a binder and filler particles is either poured directly into the space containing the hollow spheres, or into a space adjacent to the hollow spheres. Then, pressure is applied such that the slurry passes around the hollow spheres filling in any voids between the spheres and such that the slurry is also forced against the fibrous material. This fibrous material allows capillary wicking of the liquid in the slurry thereby permitting the filler particles to fill the void spaces between the spheres. The fibrous material also allows controlled removal of the liquid in the slurry and allows controlled partial drying of the sphere and slurry casting. This insulating material casting is then further beat dried then fired to create a ceramic matrix material that is highly resistive to erosion and has a very low thermal conductivity.
U.S. patent application Ser. No. 09/267,237 (Merrill et al., filed on Dec. 20, 1999) described a material useful as an erosion resistant layer for turbine applications. There, closely packed hollow, geometric shapes, such as hollow spheres were mixed with binder and filler particles, cast into a mold, dried and then fired to provide abradable, porous, thermally stable seals, and the like. This system works well for making flat structures.
The present invention has been developed as an improved process of forming larger three dimensional geometric shapes, in view of the foregoing, and to address other deficiencies of the prior art. Therefore, it is one of the main objects of this invention to provide a method making a high temperature, erosion resistant coating and material which is non-shrinking, thermally insulating, in larger geometric shapes and thermally stable up to at least 1600° C., and which has constrained stabilized porosity and insulating properties, as well as controlled thermal conductivity and thermal expansion properties.